The use of packet data transmission and reception is well known in the art. Digital data in the form of a bit stream is assembled into packets, modulated using a modulation scheme and transmitted over a medium (i.e. channel). At the other end of the communications channel, a receiver tries to receive the packet and demodulate the signal back into the original bit stream.
A high level block diagram of a prior art wireless receiver suitable for packet transmission schemes such as WLAN 802.11a/g/n or MBOA-UWB is shown in FIG. 1. The packet receiver, generally referenced 160, comprises an antenna 161, RF demodulator 163, analog to digital converter (ADC) 165, data demodulator 169 and packet detector 167.
In operation, the RF signal 162 is received (picked up) by the antenna 161 and then demodulated by an RF demodulator 163 into a base band or intermediate frequency (IF) signal 164. An analog to digital converter (ADC) 165 translates the analog signal into digital samples 166. The samples are then input to a packet detector 167, which functions to detect when a packet starts and in response provides a synchronization or sync signal 168 to a data demodulator 169. The output of the data demodulator (i.e. modem receiver) 169 translates the digital samples 166 along with the synchronization signal 168 into an output bit stream 170.
Note that the packet receiver 160 highlights the importance of the packet detector 167. The detector 167, however, is an inherently weak link in this scheme because accurate packet detection (which is usually measured by detection and false alarm probabilities) is a necessary but insufficient condition for packet detection. Therefore, if a packet is not detected, the data demodulator 169 will not be able to correctly demodulate the digital samples 166 and generate the packet in output bit stream 170.
Since the wireless medium along with other media types (e.g., cable transmission, optical fiber transmission, etc.) usually consists of noise and other impairments added to the desired signal, it is essential that the packet detector 167 be as robust as possible to the various impairments and be able to accurately provide the sync signal 168 at well below the detection conditions of the data demodulator 169 in order that it does not limit the receiver performance.
A diagram illustrating a typical prior art packet structure and associated synchronization sequence internal structure is shown in FIG. 2. A packet, generally referenced 1205, comprises modulated signals containing a sync sequence field 1201, signaling sequence field 1202 providing information about the packet length, type, priority, etc. and the actual data field (i.e. payload) 1203. The sync sequence is shown expanded into a plurality of sample repetitions. The sync sequence typically comprises periodic or spread spectrum signals comprising M repetitions 1207 of N samples 1206.
For clarity sake we denote the sync sequence 1201 samples as follows. Let b0, b1, . . . , bN−1 denote the basic sequence (i.e. spreading sequence) multiplied by spread sequence a0, a1, . . . , aM−1. The sync sequence 1201 can then be denoted by: a0b0, a0b1, . . . a0bN−1, a1b1b0, a1b1, . . . , a1bN−1, . . . , aM−1b0, aM−1b1, . . . , aM−1bN−1. We also denote cn=a└n/M┘bn mod N as the nth sample in the sequence (nε{0 . . . (N−1)(M−1)}). For brevity we denote K=NM as the length of the entire synchronization sequence 1201.
Two current wireless standards that match this pattern of packet detection are the 802.11a/g standard wherein M=10, N=16 and an=1 and the Multiband OFDM Alliance (MBOA) Ultra Wideband (UWB) standard wherein M=16 and N=8.
Packet detection of an UWB OFDM signal is based on detection of a sequence of preamble symbols. A preamble sequence may be approximated by spreading a 16-bit sequence (‘a’ sequence) by n 8-bit sequence (‘b’ sequence). High coding gain of UWB implies a high sensitivity requirement (i.e. below 0 dB SNR), while wide bandwidth and wireless channels implies immunity to the multipath conditions. In addition, in this case symbol based detection is necessary due to possible frequency offset and to lower required circuit area due to avoiding the use of large area memory and minimization of computations.
The 802.11a/g standards operates in 2.4 GHz and 5 GHz bands, respectively, and use 52-subcarrier orthogonal frequency-division multiplexing (OFDM) with a maximum data rate of 54 Mbps. Out of the 52 OFDM subcarriers, 48 are for data and 4 are pilot subcarriers. Each of these subcarriers can have the following modulations: BPSK, QPSK, 16-QAM or 64-QAM. Each of the subcarriers can be represented as a complex number wherein the actual generation and decoding of orthogonal components is performed during baseband processing typically using DSP or other processors which is then upconverted to the RF frequency at the transmitter. The time domain signal is generated by taking an Inverse Fast Fourier transform (IFFT) of the baseband transmit signal. In the receive direction, the receiver downconverts the received samples and performs an FFT to retrieve the original coefficients.
A traditional UWB transmitter works by transmitting pulses across a very wide spectrum of frequency (e.g., typically over a bandwidth of several GHz). At the receiver, the pulses are translated into data by detecting a known pulse sequence (i.e. sync sequence) sent by the transmitter. Modem UWB systems use modulation techniques such as Orthogonal Frequency Division Multiplexing (OFDM) to occupy such wide bandwidths. MultiBand OFDM is capable of dynamically turning off subbands and individual OFDM tones. This provides for good coexistence with narrowband systems such as 802.11a, adaptation to different regulatory environments, future scalability and backward compatibility.
In MultiBand OFDM, the available spectrum of 7.5 GHz is divided into several 528 MHz frequency bands. This allows the selective implementation of bands at certain frequency ranges while leaving other parts of the spectrum unused. The MBOA proposal provides for five logical channels wherein Channel 1, which contains the first three bands, is mandatory for all UWB devices and radios.
OFDM is operative to modulate the information transmitted on each band. OFDM distributes the data over a large number of carriers spaced apart at precise frequencies. This spacing provides the orthogonality, which prevents the demodulators from seeing frequencies other than their own. The benefits of OFDM are high-spectral efficiency, resiliency to RF interference and lower multipath distortion.
Two well known prior art packet detector schemes are shown in FIGS. 3 and 4. A block diagram illustrating a prior art technique for detecting synchronization sequences using cross correlation is shown in FIG. 3. The cross correlation based packet detector, generally referenced 200, comprises a delay line 202 comprising a plurality of delay elements 201, 203, 205, 207, 209, a plurality of multipliers 229, 231, 233, 235, summer 221, absolute value squared 281 and decision logic 223.
In operation, the incoming signal 280 undergoes a delay via delay line 202. The delayed signal samples are multiplied using a plurality of multipliers 229, 231, 233, 235 with the flipped conjugated synchronization sequence samples cK−1*, cK−2*, . . . , c1*, c0* 211, 213, 215, 219. The product results of the multiplications are summed together via summer 221. The total correlation result undergoes absolute value 281 and is compared against a threshold 225 using decision block 223 (e.g., a comparator). If the absolute value correlation value goes above threshold 225 the detection signal 227 becomes active indicating detection of a packet. This method is equivalent in signal processing to a matched filter achieving the best signal to noise ratio under an Additive White Gaussian Noise (AWGN) regime.
A major drawback of the prior art cross correlation scheme is that if the signal has undergone a linear multipath (frequency selective) channel (which is almost always the case in wireless and wire line transmission) then the correlation peaks become distorted and the threshold may sometimes never be crossed resulting in a high miss-detection probability (i.e. false-negatives). Lowering the threshold would result in a corresponding increase in the probability of false alarm (i.e. false-positives), just as problematic. Therefore, this makes this prior art cross correlation detection scheme is not very robust to channels.
Yet another major drawback of this prior art scheme is its lack of tolerance to carrier frequency offset. Since the local oscillator (LO) frequency generation mechanism at the transmitter and receiver are unsynchronized, the resultant signal seen by the receiver at the ADC input is frequency shifted with respect to the ideal IF frequency. This causes the delayed signal samples 280, 272, 273 and 275 to accumulate a phase rotation such that even without the presence of noise or multipath fading there would be a different phase for all correlation results summed via summer 221. This non-coherency causes the absolute value of the correlation peak to drop, thus further degrading the performance of this detector.
A block diagram illustrating a prior art method for detecting a synchronization sequence using a delay and correlate scheme is shown in FIG. 4. The delay and correlate based packet detector circuit, generally referenced 1400, comprises a delay 1401, complex conjugate block 1403, multiplier 1404, absolute value block 1405, low pass filter 1406 and decision logic 1407.
In operation, the incoming signal 1402 undergoes a delay by N using delay circuitry 1401. The original incoming signal 1402 is conjugated and then multiplied with the delayed signal 1410 using multiplier 1404. The result of this multiplication undergoes an absolute value via block 1405 and is then filtered by digital low-pass filter 1406, such as a moving average (MA) or a single order infinite impulse response (IIR) filter. The output of the filter then undergoes a decision process via decision logic 1407 which generates the detection signal 1408. The decision logic may function, for example, by comparing the result to a threshold 1420.
The principle of operation for a delay and correlate packet detector such as that of circuit 1400 is that it uses the periodicity property of the synchronization sequence to verify that the synchronization sequence is actually present at the input. Therefore, multiplying the conjugated signal with its delayed version (where the delay is equal to the synchronization signal period) should yield the absolute value of the signal, since ideally the signal and its delayed version are identical. Furthermore, a linear channel does not change the period of the signal (i.e. its frequency) but only its shape (i.e. the signal undergoes scaling, phase shifts, etc.) and therefore this detector is very robust to channel effects. This scheme is also advantageous due to its resilience to carrier frequency offset. The latter effect causes the delayed signal samples 1410 and the incoming signal samples to have a constant phase difference between them. This constant phase difference is reflected in the input to magnitude block 1405 and is thereby eliminated.
The major drawback of this detector is when white Gaussian noise (AWGN) is added to the signal (which is the case in real world channels). In the case of AWGN added to the signal, the noise samples of the incoming signal and its delayed version are uncorrelated. This causes the noise to be multiplied via multiplier 1404. Although multiplication of uncorrelated noise may be tolerable for high signal to noise ratios (SNRs), it is detrimental for signals with low SNRs where the performance of this detector quickly deteriorates.
It is thus desirable to have a packet detection mechanism that does not suffer from the disadvantages of the prior art cross correlation and delay and correlate based packet detectors. The mechanism should provide packet detection and carrier frequency offset estimation capabilities that are robust to operation in linear channels as well as to transmission signals having low SNRs.